The subject invention concerns a method of manufacturing a measuring device. The invention also concerns a measuring device produced in accordance with the method.
A principal application of a device in accordance with the present invention is to measure the pressure in combustion engine cylinders. To optimize the efficiency of the engine it is desirable to be able to instantaneously measure the pressure of the exhaust gases inside the cylinder compression space. It would be advantageous to incorporate a device designed for this purpose in the existing ignition and fuel injection system of the vehicle which, with the aid of electronic means of suitable configuration, could result in more carefully controlled fuel injection conditions and a more advantageous combustion process. Consequently, it would be possible to achieve a more optimized ignition point and to obtain more complete fuel combustion with consequential improvement of the engine efficiency.
High pressures and temperatures and serious electrical disturbances are characteristic of the particularly extreme environmental conditions to which a pressure sensor that is positioned in the cylinder is exposed. In addition, pressure sensors of this type are to be installed in connection with the manufacturing process in a large number of vehicles produced in large series and thus be incorporated as one component of the electric system of the vehicle. A pressure sensor of this kind therefore must meet severe requirements with respect to accuracy, reliability and, not the least, low manufacturing costs and satisfactory reproducibility in the manufacturing process.
In view of the environmental conditions in the cylinder it is difficult to effect pressure measurements based on direct conversion to electrical quantities. A solution based on fiber optics according to which the sensor body consists mainly of silicon and the conversion of the signal is from pressure to a mechanical quantity which in turn is converted into an optical quantity transported by an optical fiber, would provide the advantages required to solve the measuring problem. However, to date no sensors exist that are able to withstand the environmental conditions at a cost that is acceptable from a production-economical point of view.
Relevant devices in this aspect are pressure sensors based on the principle that the surrounding pressure affects the thickness of a cavity formed in the sensor and wherein optical interference is generated inside the cavity when incident light hits the cavity via an optical fiber. The method is based on the use of a resonator of the kind known as a Fabry-P erot resonator. A fiber-optical device of this nature for pressure measurements is previously known from the German Patent Specification DE 3611852. This prior-art device includes a Fabry-P erot resonator which is positioned at the end of an optical fiber. An optical signal supplied to the fiber is reflected by the Fabry-P erot resonator at an intensity that is dependent on the pressure affecting the resonator.
However, the device in accordance with this German publication is intended primarily for measuring the blood pressure in the human body, and as such it is devised for comparatively low pressures, in the range of 1 bar, and for temperatures in the range of 20.degree.-40.degree. C., i.e. close to room temperatures. The pressure to be measured inside a combustion engine cylinder, on the other hand, may amount to 200 bars at normal operational temperatures of 250.degree. C. In addition to the requirements for productional efficiency as mentioned above, a sensor of this kind therefore has to meet high demands also as regards material, manufacturing technique and reliability, and the sensor disclosed in DE 3611852 fails to do so.
Another disadvantage inherent in the device mentioned above concerns the nature of the attachment of the optical fiber included therein to the sensor body. The sensor body is said to be Joined to the fibre by means of clamping. This securement method as well as more permanent mounting methods, such as glueing, are not, however, sufficiently strong or resistant to pressure and heat to fulfill the demands that devices intended for applications of the kind contemplated by the present invention must meet. In addition, the layer of glue that results from glueing an optical fiber to the sensor body will disturb the path of the light rays into and out of the sensor body.